Automobile manufacturers and component makers have been engineering automotive components to achieve the gradually increasing Corporate Average Fuel Economy (CAFE) standards, which target an average fleet fuel consumption of 34.1 MPG by 2016 and 56.2 MPG by 2025. One design initiative to achieve CAFE standards is the reduction in the coefficient of friction of moving parts. In addition to achieving CAFE standards, a reduction in the coefficient of friction may also reduce wear and improve reliability of moving components.
One such moving part is the piston ring. One or more piston rings are commonly provided in grooved tracks around the outer perimeter of an engine piston. Where multiple rings are present, the rings may be designed to perform different or overlapping functions. For example, piston rings may be designed to seal the combustion chamber to trap combustion gasses, improving engine efficiency. Piston rings may also be designed to aid in heat transfer and manage engine oil in the cylinder.
Piston rings are often formed from a base material of cast iron or rolled carbon steel and may be coated with relatively hard, wear resistant coatings, such as nitride coatings, exhibiting hardness 2 to 4 times that of the base materials. Chromium nitride coatings may exhibit relatively low internal stress allowing for relatively thick coating layers. Chromium nitride coating deposition rates are considered somewhat favorable for production at deposition rates of approximately 2 microns per hour. Titanium nitride coatings have also been examined. However, titanium nitride coatings may exhibit relatively high internal stress compared to chromium nitride coatings.
The coefficient of friction for chromium nitride and titanium nitride coatings may be in the range of 0.5 to 0.7 in dry sliding as measured by typical pin-on-disc testing. Reducing the coefficient of friction values between a piston and cylinder liner wall may reduce overall engine friction and improve fuel efficiency. However, maintaining relatively high deposition rates, relatively high hardness and relatively high wear resistance is also desirable. Accordingly, a need for providing a relatively hard, wear resistant, and cost effective piston ring coating exhibiting a relatively lower coefficient of friction still remains.